Electrophoretic coating bath and process wherein solvent and base concentractions are controlled



c. E. COATES ELECTROPHORETIC COATING BATH AND PROCESS WHEREIN SOLVENT AND BASE CONCENTRATIONS ARE CONTROLLED Filed Feb 13, 1967 INV'ENTOR c. E COATES 7% W E M ATTORNEYS United States Patent F 3,463,712 ELECTROPHORETIC COATING BATH AND PROC- ESS WHEREIN SOLVENT AND BASE'CONCEN- TRATIONS ARE CONTROLLED Carlton E. Coates, Savage, Minn., assignor to Ashlantl Oil 8; Refining Company, Ashland, Ky., a corporation of Kentucky Filed Feb. 13, 1967, Ser. No. 615,555 Int. Cl. C23b 13/00; 301k 5/02 U.S. Cl. 204181 7 Claims ABSTRACT OF THE DISCLOSURE Electrophoretic coating employing a two-phase bath comprising:

(A) a discontinuous phase comprising (1) an uncured, curable, electrodepositable material (2) a solvent for said material, and

(B) a continuous phase comprising a solution of the solvent in the bath liquid wherein the solubility of the solvent in the bath liquid is less than the solubility of the bath liquid in the solvent.

In recent years the use of electrophoretic coating processes has increased greatly in industry and in particular in the sheet metal industry in cases in which it is desirable to provide the sheet metal with a protective coating. Electrophoretic coating processes possess a number of advantages not found in spray coating, especially in the cases where the substrate to be coated is a sheet metal object having intricate curves such as automotive bodies and metal furniture. In spray coating, the coating material, directed from a point source such as a spray gun, impinges upon the substrate to be coated. In such processes considerable difiiculty is encountered in coating inaccessable areas, where as in the electrophoretic coating processes the entire substrate to be coated is immersed in a bath in which an electrodepositable material has been dispersed. The substrate to be coated is made an electrode and a difference in potential is created between the substrate to be coated and another electrode immersed in the bath. This diiference in potential causes the electrodepositable material to migrate towards the substrate and deposite itself thereon. The coated substrate is then removed from the bath, optionally Washed, and the coating cured by processes such as heating. In order to operate these electrophoretic coating processes continuously the electrodepositable material is added either continuously or at intervals, in the form of a coating composition. This coating composition comprises the electrodepositable material, and a solvent for the electrodepositable material. In addition the coating composition frequently contains another liquid such as the liquid of the bath which is frequently Water. In the usual case in which the water is insoluble or only partially soluble in the solvent the coating composition is in the form of a dispersion. These coating compositions frequently contain a material such as a base which increases the water dispersibility of the electrodepositable material. However, in operation the solvent for the electrodepositable material and the base when present, tend to dissolve in the bath. As the coating operation continues the concentration of the solvent and the base in the bath inice creases. Increased concentrations of these materials in the bath alters the electrical characteristics of the bath, creating a number of undersirable results. For example, process control becomes difiicult, increased amounts of base renders the bath more conductive and requires higher amperage with increased gassing.

It is therefore an object of the present invention to provide a novel electrophoretic coating bath which overcomes the disadvantages of the prior art.

Another object of the present invention is to provide a novel electrophoretic coating process which overcomes the disadvantages of the prior art.

A 'further object of the present invention is to provide a novel electrophoretic coating bath and process for using the same in which there is no increaes in concentration inthe coating bath of the solvent used in the coating composition.

A still further object of the present invention is to provide a novel electrophoretic coating bath and process for using the same in which the increase in concentration isthe bath of the base used in the coating composition is reduced.

Yet another object of the present invention is to provide a novel method for eliminating the build up on the bath of the solvent for the electrodepositable material.

Additional objects and advantages of the present invention will be apparent by reference to the following detailed description and the single figure of the drawing wherein there is schematically shown a preferred embodiment of the present invention.

Referring now to the drawing there is schematically shown an apparatus suitable for practicing a method of the present invention and in which the novel bath of the present invention can be advantageously employed. The apparatus comprises a bath tank 11 adapted to hold an electrophoretic coating bath 12. In electrically conductive contact with the bath 12 is an electrode 13, eonnected to a power source 14 by means of a conductor 15. A substrate 16 is immersed in the bath 12. The power source 14 is electrically connected to the substrate 16 by means of conductor 17. While the substrate 16 can be either the anode or the cathode, in a preferred embodiment of the present invention the power source 14 is arranged such that the electrode 13 is the cathode and the substrate .16 is the anode. A coating composition 18 is contained in a tank 19 which is in liquid communications with a pump 20 adapted to supply the coating composition to the bath tank 11 from the tank 19. The bath tank 11 can be provided with a weir type overflow 21.

Prior to setting the apparatus in operation the coating compositions employed in the present invention are formulated as described more completely below. These coating compositions comprise an uncured curable electrodepositable material and a solvent for the material. While it is preferred to employ solvents which are completely miscible with the electrodepositable material any solvent which is otherwise suitable and is soluble in the material to an extent sufficient impart the desired viscosity to the mixture can be employed. The choice of this solvent is critical to the present invention. The solubility of the solvent in the bath liquid must be less: than the solubility of the bath liquid in the solvent. Further, the bath liquid (usually water) must be partially soluble in the solvent. Solubilities of the bath liquid in the solvent should desirably be greater than one percent (e.g. greater than 5%) by weight at 20 C. Whether a solvent possesses the above described solubility characteristics can be easily determined by conventional procedures well-known in the art. For instance, in a preferred embodiment of the present invention in which the bath liquid comprising the bath 12 is Water the respective solubilities of th solvent in water and water in the solvent can be determined by adding each to the other until a second phase separates out. Since solubilities are temperature dependent, these tests are conducted at the ultimate desired temperature for the bath 12. The same procedure is em loyed when the bath liquid is any liquid other than water or when the bath liquid is a mixture. Examples of suitable solvents which can be employed in the present invention are listed in Table I.

TABLE I Solubility of the Solubility of water solvent in water in the solvent (percent by (percent by 1 This solvent is commercially available under the trade name Pro' pasol B. Mixtures of these solvents can also be employed. The most preferred solvent 15 n-butanel.

Substantially completely water miscible solvents, i.e. those with a solubility of water in them of 100 percent by weight or more (eg. infinity), are unsuitable for use in the present invention. Examples of these unsuitable so]- vents include among others aliphatic alcohols having l3 carbon atoms such as methanol, ethanol, propanol, isopropanol, t-butanol, ethylene glycol and propylene glycol; the hydroxy ethers such as 2-ethoxyethanol commercially available as Cellosolve, Z-butoxyethanol commercially available as Butylcellosolve, and bis(2-ethoxyethyl) ether commercially available as D-iethyl Carbitol.

Also unsuitable for use in the present invention are solvents which have a solubility in water greater than the solubility of water in them. Examples of these solvents are given in Table II.

TABLE II Solubility of the Solubility of Water solvent in water in the solvent 1 This product is commercially available as Cellosolve acetate. 2 This product is commercially available as Peutroxone solvent.

The preferred uncured curable electrodepositable materials employed in the present invention are the carboxylic acid resins such as acrylic, epoxy or m st preferably polyester resins. These resins can be produced by known process such as those described in Gilchrist U.S. Patent 3,230,162 and French Patent 1,377,207.

The polyesters useful in the present invention can be prepared by known esterification reactions under conditions well-known in the polymer art by reacting a polyol and a polycarboxylic acid in a stoichiometric ratio of 1:2 to 2:1 and preferably 121.1 to 1.1:1 at a temperature between about 15 C. and 150 C. until the acid number, a value well-known in the polyester art, of the reaction mix is reduced from its initial value to a value of from about 20 to 200, which generally occurs within five minutes to ten hours. At acid numbers much below 20, the polyester tends to have poor dispersability and at acid numbers much above 200 the molecular weight of the polyester is tool low to cure to a suitable film. The preferred polyesters have an acid number of from 30 to 120.

The polycarboxylic acids which can be reacted with polyols to give polyesters useful in the present invention can be saturated, unsaturated, aliphatic, cycloaliphatic, aromatic, or heterocyclic. Examples of these include, among others, phthalic acid, isophthalic acid, terephthalic acid, di-, tetraand hexahydrophthalic acid, 3-brom0- phthalic acid, 3,4-dibromophthalic acid, 3,4,5,6,7,7-hexachloro-3,6-endomethylene-l,2,3,6-tetrahydrophthalic anhydride (chorendic anhydride), succinic acid, maleic acid, chlorosuccinic acid, monochloromaleic acid, 6-ethyl-4- cyclohexadiene-l,Z-dicarboxylic acid, 3,6-dimethyl-4-cyclohexadiene-l,2-dicarboxylic acid, 6-buty'l-3,5-cyclohexadiene-1,2-dicarboxylic acid, octadecylsuccinic acid, dodecylsuccinic acid, dioctylsuccinic acid, nonadecadienylsuccinic acid, 3-methoxy-1,2,3,6-tetrahydrophthalic acid, polyadipic acid, polysebacic acid, polyisophthalic acid, dimer fatty acid, fumaric acid, oxalic acid, malonic acid, succinic acid, adipic acid, itaconic acid, their corresponding extant anhydrides, and mixtures thereof.

The polyols which can be reacted with the polycarboxylic acids in order to give the polyester useful in the present invention are those which have heretofore been found to be reactive with polycarboxylic acids. Examples of these include, among others, ethylene glycol, diethylene glycol, 1,2-propylene glycol, 1,2- or 1,3-dipropylene glycol, 1,3-propylene glycol, 1,3-butylene glycol, 1,2-butylene glycol, neopentyl glycol, 1,3-pentanediol, 1,5-pentanediol, trimethylol propane, pentaerythritol and mixtures thereof. The preferred polyols are those just listed which are aliphatic, dihydric alcohols of 1 to 8 carbon atoms. Higher alcohols can be employed in minor amounts as is wellknown in the polyester art.

The polyesters useful in the present invention can be of the alkyd resin type. These alkyd resins can be produced according to procedures well-known in the art by incorporating an oil or a fatty acid into the polyester. Examples of suitable oils include, among others, linseed, tung, castor, dehydrated castor, safflower, soya, tall, cottonseed, olive and cocoanut oils. Examples of suitable fatty acids include among others myristic, palmitic, stearic, palmitoleic, oleic, linoleic, caprylic, capric, lauric, linolenic, oleostearic, and ricinoleic acids. Other monobasic acids such as benzoic acid and p-tert-butyl benzoic acid can also be used.

The water dispersibility of the resins useful in the present invention can be enhanced by mixing them with a base. Although inorganic bases such as potassium hydroxide and ammonia can be used in batch processes the organic bases are preferred for both batch and continuous processes. Examples of suitable organic bases include among others hydroxyamines, polyamines and monoamines such as monoethanolamine, diethanolamine, tricthanolamine, N-methyl ethanolamine, N-aminoethylethanolamine, N-methyl diethanolamine, monoisopropanolamine, triisopropanolamine, polyglycol amines such as HO(C H O) C H NH hydroxylamine, butanolamine, hexanolamine, methyldiethanolamine, octanolamine, and alkylene oxide reaction products of monoand polyamines such as the reaction product of ethylene diamine with ethylene oxide or propylene oxide, laurylamine with ethylene oxide or propylene oxide laurylamine with ethylene oxide; ethylene diamine, diethylene triamine triethylene tetramine, hexamethylene tetramine, tetraethylene pentamine, propylene diamine, 1,3-diaminopropane, imino-bis-propyl amine; mono-, diand tri-lower alkyl amines having 1 to 8 carbon atoms, such as mono-, diand tri-ethyl amine.

The above described bases and especially the amines have generally been employed in the prior art in a substantially stoichiometric amount, easily determined by titrating a sample of the carboxylic acid resin with potassium hydroxide in a manner similar to that employed in acid number determinations as is well-known in the art. The polycarboxylic acid resin is then mixed with an amount of the amine stoichiometrically equivalent to the potassium hydroxide employed in the titration. In the present invention a less than stoichiometric amount of the amine base can be employed. While this unexpected property is not completely understood it is apparently due to the surfactant properties of the solvent which aids in the water dispersibility of the carboxylic acid resin and reduces the amount of amine necessary. Since less amine is employed in the coating composition the rate of build up of amine in the bath 12 is reduced.

While the coating composition 18 can be added to the bath 12 in the form of a mixture of the solvent and the electrodepositable material it is preferably added in the form of a water dispersion having a composition similar in most respects to the bath 12 but having a higher concentration of the depositable material. This dispersion is formed by adding the bath liquid, usually water, to a mixture of the solvent, the electrodepositable material, and the base. As the water is added a certain amount dissolves in the mixture, after the mixture becomes saturated with water the water forms a separate discontinuous phase. With continued addition of water the phase relationship of the dispersion inverts at which point the water becomes the continuous phase and the mixture of the electrodepositable material, the solvent, and the base takes the form of droplets and becomes the discontinuous phase. The solvents employed in the present invention aid in this inversion process and permit inversion to take place at a much lower base content and a much lower water content than heretofore possible. The coating compositions employed in the present invention can be variously described as emulsions or dispersions. Some are milky and opaque whereas others are clear, however most of the clear coating compositions are colloidal in nature as evidenced by the visibility of a beam of light shined through the compositions at right angles to the line of sight.

In operation the substrate 16 is fed in the direction of the arrow 22 into the bath 12 from any suitable source such as roll 23. The power source 14 is arranged such that a potential difference of 50 to 500 volts is created between the substrate 16 and the electrode 13. In the preferred embodiments of the present invention wherein the electrodepositable material is a carboxylic acid resin the power source 14 is arranged such that the substrate 16 is the anode and the electrode 13 is the cathode. By virtue of this potential difference the droplets'which constitute the discontinuous phase supplied to the bath from the coating composition 18 migrate towards the substrate 16 and are deposited thereon. Because of the above-described solubility characteristics of the solvent, the bath 12 rapidly becomes saturated with the solvent. At this point the solvent is entrained in the discontinuous phase and deposits on the substrate 16. The solvent is thus removed from the bath as an integral part of an uncured wet coating 24 on the substrate 16. Because of continual removal of the solvent, build up of the solvent in the bath 12 is eliminated. The substrate 16 now having the uncured wet coating 24 leaves the bath 12 for further conventional processing steps such as washing and heating to drive off the solvent and cure or cross-link the electrodepositable material.

The present invention is not only applicable to continuous processes such as that described above but also to batch processes. Continuous processes comprehend not only those described above wherein the substrate comprises a web of indefinite running length but also process in which separate parts such as automobile doors are conveyed by means such as an overhead conveyor and dipped into the bath. See Burnside et al. U.S. Patent 3,200,057. The present invention is equally applicable to processes in which the substrate 16 is the cathode and the coating composition contains a cationic paint which will deposit on the cathode.

The quantities of the components in the novel electrophoretic coating baths can vary widely.

The electrodepositable material i.e. resin or combination of resins and/or cross-linking agents) comprises from .001 to 50 and preferably from 3 to 15 percent by weight of the bath. Substantially all this material is con centrated in the discontinuous phase.

The base can comprise from 0.05 to 2 percent by weight of the bath. It is distributed between the continuous and discontinuous phases depending on the nature and solubility of the base and the presence or absence of other reactive components such as acids and in particular fatty monocarboxylic acids such as lauric acid which react with amine and are used to keep the pH of the bath at any desired or pre-selected value. As is well-known in the art the pH of the bath can vary widely but is preferably kept between 5 and 9.

The bath can also contain widely varying amounts of other non-interfering material in amounts up to and in excess of 20% by weight of the bath. Examples of these materials include the reaction products of the base and the monocarboxylic acid, water miscible solvents, surfactants, pigments and extenders.

The quantity of the solvent for the electrodepositable material can also vary widely but generally comprises from 0.01 to 50 percent and preferably from 0.1 to 10 percent by weight of the bath. The solvent present in the bath is distributed between the continuous and discontinuous phases depending upon factors such as the solubility of the solvent in the bath liquid, the solubility of the bath liquid in the solvent, the presence or absence of the above described non-interfering materials, the pH of the bath and the amount and type of base present. In general the weight percent of solvent in the continuous phase of the bath is less than the solubility of the solvent in the bath liquid. For example when the bath liquid is water and the solvent is n-butanol, the continuous phase of the bath will generally contain no more than 7.7 parts by weight of n-butanol per 100 parts by weight of the continuous phase of the bath.

As previously indicated the bath is typically prepared by adding a prepared coating composition or dispersion thereof to a bath or bath liquid. The quantities of the components in the coating composition are adjusted such that the quantities of these components in the bath will remain within specified limits as the coating composition (or dispersion thereof) is added to the bath. However in general it is desirable that the weight of electrodepositable material plus any pigment that is present comprise from 10 to 95 percent and preferable from 60 to percent by weight of the weight composition. The solvent is generally present in an amount sufficient to give the resultant mixture of solvent and electrodepositable material (plus usually a pigment) the desired viscosity and generally comprises from 5 to 90 percent and preferably from 10 to 40 percent by weight of the coating composition. As previously described the base, if any, is generally present in the coating composition in less than a stoichiometric amount and comprises from 0.1 to 10 and preferably from 1.0 to 6.0 percent by weight of the coating composition.

When it is desired to add the coating composition to the bath or bath liquid in the form of an aqueous dispersion it is necessary to first add water to the coating composition until inversion of the phases takes place. In this case, the amount of water present in the dispersion is generally the minimum which will form a continuous phase and generally from 10 to 900' and preferably from 25 to 500 weight percent of the coating composition. The solvents employed in the present invention favor the use of lesser amounts of water.

These coating compositions can also contain varying amounts of other non-interfering materials.

The invention may be better understood by reference to the following examples in which all parts and percentages are by weight unless otherwise indicated. These examples are illustrative of certain embodiments designed to teach those skilled in the art how to practice the invention and to represent the best mode contemplated for carrying on the invention and are not intended to limit the scope of the invention in any manner.

EXAMPLE 1 This example illustrates the synthesis of a polyester useful as a carboxylic acid resin the coating composition employed in the present invention.

The indicated quantities of the following reactants are placed in an open, scalable, reaction vessel which is capable of withstanding internal pressures of 350 p.s.i.g. and which is provided with a stirrer, a thermometer, a pressure gage, a control valve containing vapor outlet line, and an inert gas/vacuum inlet.

Reactant: Quantity in grams Once refined soybean oil 593 Glycerine 222 Benzoic acid 87.5

' Isophthalic acid 533 Bis(2-ethoxyethyl)ether 28.5 n-Butanol 307 The vessel is then closed, evacuated to 29 inches of mercury, and heated to 450 F. for 3 hours while maintaining the internal pressure between 95 and 110 p.s.i.g by manipulation of the control valve to bleed olf vapor, which consists chiefly of water of reaction formed by the esterification of the glycerine and the isophthalic acid and the bis(2-ethoxyethyl)ether which is volatile under the above reaction conditions. Heat is removed and the control valve opened reducing the pressure to Zero p.s.i.g. and the vessel sparged with carbon dioxide through the inert gas inlet. Heating is continued at 400 to 420 F. and atmospheric pressure for hours at which time the resin has an acid number of 46.8. The resin is reduced to a nonvolatile content of 80% with n-butanol. The resin solution has the following characteristics:

Non-volatile content percent 79.0 Viscosity "stokes" 61 Gardner-Holt color 4 Wt./gallon lbs./gal 8.75 Acid number solids 46.7

This resin solution (2362.5 g.) is mixed with N,N-dimethyl-ethanolamine (81.1 g.). Water is added to this mixture until the mixture inverts and forms a coating composition having a continuous water phase.

EXAMPLE 2 This example further illustrates the present invention.

An electrophoretic coating bath is formed by mixing the coating composition of Example 1 (5.38 lbs.), and water (4.91 gals.) in a bath tank equipped with a cathode. A continuous substrate of sheet steel shim stock 0.005 inch thick and 1 foot wide is fed into the bath at a rate of 1 foot per minute. The substrate is the anode and a voltage difference between the anode and the cathode of 75 volts is maintained. The amount of the coating compositions in the bath is maintained at a constant level by periodic addition of amounts of the coating composition containing amounts of the resin of Example 1 equal to the amounts of the resin removed in the uncured wet coating on the substrate. From the amount of coating composition added the number of turnovers in the bath is calculated at periodic intervals and the percent of n-butanol in the bath at these same intervals is measured. One turnover has taken place when 5.20 lbs. of resin at a non-volatile content of 80% which is equal to the amount of resin originally present in the bath, has been added while maintaining the resin content of the bath at TABLE III Linear travel of the substrate in It.

Weight percent of Number of turnovers n-butanel in the bath From the foregoing it can be seen that the percent of n-butanol in the bath remains substantially constant over the entire run and does not increase.

Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, it will be understood that variation and modifications can be effected within the spirit and scope of the invention as described above and as defined in the appended claims.

What is claimed is:

1. A liquid, two-phase, electrophoretic, coating bath comprising:

(A) a discontinuous phase comprising 1) an uncured, curable, electrodepositable material and (2) a solvent for said material, and (B) a continuous phase comprising a solution of:

(1) a bath liquid and (2) said solvent for said material wherein the solubility of the solvent in the bath liquid is less than the solubility of the bath liquid in the solvent and wherein the bath liquid is at least partially soluble but not completely soluble in said solvent.

2. The bath of claim 1 wherein said solvent is n-butanol and said bath liquid is water.

3. The bath of claim 1 wherein said uncured, curable electrodepositable material is a polyester having an acid number of 20 to 200.

4. The bath of claim 3 further comprising a base.

5. The bath of claim 4 wherein said base is an amine.

6. A process for electrophoretically coating a conductive substrate comprising the steps of:

(A) passing the substrate into and out of the bath of claim 1,

(B) electrophoretically depositing the uncured, curable, electrodepositable material on the substrate while the substrate is in the bath by passing electric current between said substrate and an electrode inserted in said bath,

(C) adding to the bath a coating composition compris- (1) an uncured, curable electrodepositable material and (2) a solvent for said material whereby the amount of the solvent in the bath remains substantially constant.

7. The process of claim 6 wherein the substrate is an anode; wherein a cathode contacts said bath; and wherein 9 said step of electrodepositing said material is effected by creating a ditference in potential between said anode and said cathode.

References Cited UNITED STATES PATENTS 10 3,340,172 9/1967 Huggard 204-181 3,364,162 1/1968 Huggard 204-181 3,404,079 10/ 1968 Boardman 204181 5 JOHN H. MACK, Primary Examiner E. ZAGARELLA, JR., Assistant Examiner 

